JP2009-280020A discloses an air-conditioning device that includes: a thermal fluid heater that heats blown air to be blown into a vehicle cabin; an electric heater that heats heating medium circulating through the thermal fluid heater; and a heat pump that circulates cooling medium through a cooling medium/heating medium heat exchanger that heats the heating medium.
With the above-described air-conditioning device, at the cabin-heating time when the outside-air temperature is about 0° C., the cabin heating is performed at high-power by operating both of the electric heater and the heat pump. On the other hand, at the cabin-heating time when the outside-air temperature is higher than 0° C., the cabin heating is performed at low-power by stopping the operation of the electric heater and by operating the heat pump only.